1. Field of the Invention
This invention relates to a method for preparing a biodegradable copolyester, more particularly to a method including a step of preparing an aromatic prepolymer in the preparation of a biodegradable copolyester, and the biodegradable copolyester prepared therefrom.
2. Description of the Related Art
Typical polyesters used for various products including synthetic fibers, moldings, formings, plastic films and textiles, etc., are high molecular weight aromatic polyesters. These aromatic polyesters are produced by esterification and condensation of terephthalic acid with ethylene glycol or terephthalic acid with butylene glycol. Although the aromatic polyesters thus formed have good processability, they are not degradable naturally and tend to cause environmental pollution.
For the purpose of environmental protection, many attempts have been made to use aliphatic polyesters instead. The aliphatic polyesters are biodegrable and will not pollute the environment. However, applications of the aliphatic polyesters have been limited because of their low heat resistance and unsatisfactory mechanical properties. In order to obtain polyesters that have both good physical properties and processability, it has been attempted to introduce an aryl functional group into molecules of the aliphatic polyesters, so as to impart biodegradable property to the aliphatic polyesters. Nevertheless, the biodegradable rate of the aliphatic polyesters decreases with an increase in the amount of the aryl group introduced to the molecules of the aliphatic polyesters.
U.S. Pat. No. 6,020,393, assigned to Eastman Chemical Company, describes a biodegradable random branched aliphatic-aromatic copolyester, which was prepared by transesterification of an aromatic diester, an aliphatic diester, and an aliphatic glycol in the presence of a branching agent, at a temperature ranging from 190° C. to 210° C., so as to obtain a transesterified product. Polycondensation of the transesterified product was then conducted at 250° C., under a vacuum of approximately 0.1 mmHg, so as to form the aliphatic-aromatic copolyester. The branching agent was used for increasing the length of the molecular chain of the aliphatic-aromatic copolyester. However, the biodegradable rate and elongation property of the copolyester decreased with an increase in the amount of side chains in the molecules of the copolyester.
U.S. Pat. No. 6,414,108, assigned to BASF Aktiengesellschaft, describes biodegradable polyether esters which were prepared by reacting adipic acid with 1,4-butylene glycol at a temperature ranging from 230° C. to 240° C., so as to form a prepolymer, and then adding dimethyl terephthalate and 1,4-butylene glycol to the prepolymer, so as to carry out polycondensation in a reduced pressure.
U.S. Pat. No. 6,399,716, assigned to IRE Chemical Ltd., describes a biodegradable copolyester resin composition prepared by the following four steps: (1) preparing an aliphatic prepolymer; (2) reacting the prepolymer with an aromatic dicarboxylic acid and an aliphatic glycol at a temperature ranging from 180° C. to 220° C.; (3) adding an aliphatic dicarboxylic acid to the reaction mixture at a temperature ranging from 150° C. to 180° C.; and (4) conducing polycondensation of the reaction mixture at a reduced pressure.